1. Field of the Invention
This invention relates to an air-fuel ratio control device for an engine capable of pertinently controlling an air-fuel ratio by compensating variation of the air-fuel ratio sensor.
2. Discussion of Background
Explanation will be given to a conventional air-fuel ratio control device for an engine of this kind, referring to FIGS. 15 to 20. First, explanation will be given to FIG. 18. FIG. 18 is a construction diagram showing construction of a speed density type fuel injection device.
In FIG. 18, an engine 1 mounted on, for instance, a vehicle, sucks air from an air cleaner 2 through an intake pipe 3 and a throttle valve 4.
In the ignition time, an ignitor 5 is switched from ON to OFF, for instance, by a signal from a signal generator (not shown) in a distributor. By this switching, a high tension ignition signal is generated on the secondary side of an ignition coil 6, which is supplied to an ignition plug (not shown) of the engine 1 thereby performing the ignition.
In synchronism with the generation of this ignition signal, fuel is supplied by injection from an injector 7 to the inner portion of the intake pipe 3 on the upstream side of the throttle valve 4. The fuel supplied by injection is sucked to the engine 1 by the above sucking operation.
Exhaust gas after combustion is exhausted outside of the system through an exhaust manifold 8 and a three way catalytic convertor 14.
In this three way catalytic convertor 14, an air-fuel ratio having high purification ratios of three components of NOx, HC and CO in the exhaust gas is in the neighborhood of a domain wherein the air excess ratio is as .lambda.=1, that is, at a theoretical air fuel ratio. As shown in FIG. 20, in the purification ratio characteristic, the purification ratios of all the three components of NOx, HC and CO are high when the air-fuel ratio is the theoretical air-fuel ratio (.lambda.=1), the purification ratios of HC and CO are worsened when the air-fuel ratio is RICH (.lambda.&lt;1), and the purification of NOx is worsened when the air-fuel ratio is LEAN (.lambda.&gt;1).
In the meantime, an intake pipe pressure on the downstream side of the throttle valve 4 of the intake pipe 3, is detected by a pressure sensor 9 in absolute pressure, and an analogue pressure detecting signal the size of which corresponds with the absolute pressure, is outputted.
Furthermore, an oxygen sensor 10 provided at the exhaust manifold 8 detects oxygen concentration of the exhaust gas. The oxygen sensor 10 operates normally in response with the oxygen concentration when temperature of the exhaust gas reaches an allowable temperature of 450.degree. C. to 600.degree. C. or more, and outputs an analogue concentration detecting signal corresponding with the air excess ratio .lambda., as shown in FIG. 20.
The analogue pressure detecting signal, the analogue concentration detecting signal and a primary side signal of the ignitor 5 are inputted to a control device 11. The control device 11 processes an operational flow of FIG. 17, when a key switch 12 is made ON and the control device 11 is supplied with power from a battery 13, calculates fuel injection quantity corresponding with running condition of the engine 1, and performs a valve opening control of the injector 7.
FIG. 19 shows a block construction of the control device 11. In FIG. 19, a reference numeral 100 designates a microcomputer, which is composed of a CPU 200, a counter 201, a timer 202, an A/D (analogue/digital) converter 203, a RAM 204, a ROM 205 which stores a program of an operational flow of FIG. 17, an output port 206, a bus 207 and the like.
A primary side ignition signal from the ignitor 5 is shaped by a first input interface circuit 101, which is inputted to the microcomputer 100 as an interruption input signal.
At this interruption time, a measured value of the period of the ignition signal of the counter 201 is read, which is stored in the RAM 204 for detecting a revolution number.
Output signals of the pressure sensor 9 and the oxygen sensor 10 are removed with their noise components by a second input interface circuit 102, which are successively A/D-converted by the A/D (analogue/digital) convertor 203.
Fuel injection quantity is calculated in the form of the valve opening time of the injector 7 corresponding with the running condition of the engine 1, which is set by the timer 202.
In the operation of the timer 202, a predetermined level of voltage is outputted from the output port 206, which is converted from voltage to current by an output interface circuit 103, and performs the valve opening of the injector 7. Fuel is supplied by injection from the injector 7 by the valve opening.
The microcomputer 101 is operated by receiving supply of a constant voltage from a power source circuit 104 to which voltage of the battery 13 is inputted.
Next, explanation will be given to the operation of the CPU 200, referring to FIGS. 15 through 17. FIG. 15 is a control block diagram of the conventional example, and FIGS. 16A through 16E are timing charts showing the operation.
A basic pulse width T.sub.B is calculated by a basic pulse width calculating means 21 from an intake pipe pressure P detected by the pressure sensor 9 and from a revolution number 4 which is calculated by a revolution number calculating means 20 corresponding with the period of the primary side ignition signal.
On the other hand, an output voltage V.sub.02 of the oxygen sensor 10 is a signal containing a high-frequency component based on nonuniformity of the exhaust gas, as shown in FIG. 16A. As shown in FIG. 16B, a filter output voltage V.sub.02F thereof becomes a signal showing an averaged air-fuel ratio removed with the high-frequency component by passing the signal through a low-pass filter 22.
Although the low pass filter 22 may be composed of an electric circuit, the filtration can be realized by a digital filter treatment by CPU 200.
Next, the filter output voltage V.sub.02F is compared with 0.5 V by an air-fuel ratio comparing and determining means 23. As a result, as an output signal K.sub.RL (FIG. 16C), a RICH signal is outputted when V.sub.02F .gtoreq.0.5 V, and a LEAN signal, when V.sub.02F &lt;0.5 V.
An air-fuel ratio correction quantity calculating means 24 calculates an integration correction quantity K.sub.I (FIG. 16D) by integrating -.DELTA.K.sub.I when the output signal K.sub.RL is RICH, and by integrating +.DELTA.K.sub.I when the output signal K.sub.RL is LEAN. Furthermore, an air fuel ratio correction coefficient K.sub.FB shown in FIG. 16E is outputted by adding the integration correction quantity K.sub.I with -K.sub.P when the output signal K.sub.RL is RICH, and by adding the integration correction quantity K.sub.I with +K.sub.P when the output signal K.sub.RL is LEAN.
An air-fuel ratio correcting means 25 corrects the basic pulse width T.sub.B based on the air-fuel ratio correction coefficient, and outputs a pulse width T.
Lastly, an injection timing controlling means 26 performs the valve opening control in synchronism with the primary side ignition signal from the ignitor 5, during the time of the pulse width of T.
FIG. 17 shows an operational flow chart of the above operation. In Step S10 of FIG. 17, the operation calculates the revolution number N from the measured value of the period of the ignition signal, and stores it to the RAM 204.
In Step S11, the operation A/D-converts the analogue output signal from the pressure sensor 9 by the A/D converter 203, and stores it in the RAM 204 as the intake pipe pressure P.
In Step S12, the operation looks up a two-dimensional map in the ROM 205 from the revolution number N and the intake pipe pressure P, calculates a volume efficiency C.sub.EV (N,P) which is previously and experimentally obtained corresponding with the revolution number and the intake pipe pressure, and calculates the basic pulse width T.sub.B by the equation of T.sub.B =K.sub.O .times.P.times.C.sub.EV where K.sub.O is a constant.
Next, in Step S13, the operation determines whether it is on a timing at every 10 ms, and proceeds to Step S19, if not.
Furthermore, when the operation is on the timing at every 10 ms in Step S13, the operation A/D-converts the analogue output signal of the oxygen sensor 10 by the A/D convertor 203, and stores it in the RAM 204 as the sensor output voltage V.sub.02 in Step S14.
Step S15 shows a digital low-pass filter treatment, wherein the operation calculates a new filter output voltage V.sub.02F(n) by equation of V.sub.02F(n) =(1-K.sub.F).times.V.sub.02F(n-1) +K.sub.F .times.V.sub.02 from the current oxygen sensor output voltage V.sub.02 and a filter output voltage V.sub.02F(n-1) 10 ms before the current timing.
This digital filter is a primary low-pass filter, and the time constant .tau. is shown by equation of .tau.=-10/ln(1-K.sub.F)ms.
Next, in Step S16, the operation compares the filter output voltage V.sub.02F with 0.5 V. When V.sub.02F.gtoreq. 0.5 V (RICH), the operation decreases the integration correction quantity K.sub.I by .DELTA.K.sub.I in Step S17. When V.sub.02F &lt;0.5 V (LEAN), the operation increases the integration correction quantity K.sub.I by .DELTA.K.sub.I in Step S18.
After the treatments of Step S17 and Step S18, and after that of Step S13 when the operation is not on the timing at every 10 ms, the operation proceeds to Step S19, and compares the filter output voltage V.sub.02F with 0.5 V. When V.sub.02F .gtoreq.0.5 V (RICH), in Step S20, the operation stores a value of the integration correction quantity K.sub.I subtracted by K.sub.P as the air-fuel ratio correction coefficient K.sub.FB to the RAM 204. When V.sub.02F &lt;0.5 V (LEAN), in Step S21, the operation stores a value of the integration correction coefficient K.sub.I added with K.sub.P as the air-fuel ratio correction coefficient K.sub.FB.
After the treatments of Steps S20 and S21, the operation proceeds to Step S22, and calculates the pulse width T by the equation of T=T.sub.B.times.K.sub.FB, from the basic pulse Width T.sub.B and the air-fuel ratio correction coefficient K.sub.FB,stores it in the RAM 204, returns to Step S10, and repeats the above operation .
The calculated pulse width T is set to the timer 202 in synchronism with generation of the ignition signal, and operates the timer 202 during the time of the pulse width T.
As a result of the above operation, the average air-fuel ratio for the mixture is controlled so that it becomes the theoretical air fuel ratio of the air excess ratio .lambda.=1.
However, there is a time lag in a filter control system of an actual engine. The lag time from the RICH side which thicks the mixture to the LEAN side which thins the mixture, and the lag time from the LEAN side to the RICH side, are not the same, and varies also with the running condition of the engine. Therefore the averaged air-fuel ratio may be deviated from a domain wherein a high purification ratio of the exhaust gas is obtained.
Furthermore, the air-fuel ratio sensor which detects the oxygen concentration of the exhaust gas is provided at a portion of the exhaust system which is as near as possible to the combustion chamber, that is, at a gathering portion of exhaust branch pipes on the upstream side of the catalytic convertor. The averaged air-fuel ratio may be deviated from a domain wherein a high air-fuel ratio of the exhaust gas is obtained, also by variation of the output characteristic of the air-fuel ratio sensor. Causes of the variation of the output characteristic of the air-fuel ratio sensor are enumerated as follows.
(1) An individual difference of the air-fuel ratio sensor per se; PA1 (2) Nonuniformity of mixing of the exhaust gas at the position of the air-fuel ratio sensor due to tolerances of installing positions of parts installed to the engine such as the fuel injection valve, an exhaust gas recirculation valve and the like; and PA1 (3) A timewise or aged deterioration of the output characteristic of the air-fuel ratio sensor.
Furthermore, other than the air-fuel ratio sensor, the nonuniformity of the mixture of the exhaust gas due to the timewise or the aged deterioration of the engine state such as in the fuel injection valve, exhaust gas recirculating flow quantity, a tappet clearance, and variations in making thereof, may be magnified.